1. Field of the Invention
This invention relates to the field of transport photonics. More specifically the invention relates to physical layer transport testing of the integrity of optical network components.
2. Background Art
Originally, communication networks were constructed of copper wires for the transmission of electrical signals to communicate data and sound. For instance, some cable networks still transmit video and audio communication over copper wires. However, the demand created by a larger number of customers, need for increased programming choices, as well as increased communication distances, resulted in the proliferation of fiber optic transmission networks. This is because electrical signal transmission has very limited data carrying capability (i.e., low bandwidth) in contrast to optical transmission. Moreover, electrical based communication systems suffer from power losses (due to diffusion and skin effect) that accompany copper based transmission lines. Thus, in the telecommunications field, optical fibers and optical fiber cables have become the transmission media of choice, primarily because of the tremendous bandwidth capabilities and low power loss associated with optical fibers.
In current optical fiber communication systems, communication channels typically involve transmitting signals impressed on laser beams having different wavelengths propagating through optical fiber (e.g. Wavelength Division Multiplexing (WDM)). Although optical fiber communication systems utilizing such wavelength-distinct modulated channels may carry information over long distances, signals transmitted through optical fibers are attenuated and may be distorted due to the cumulative and combined effects of absorption and scattering. While the signal attenuation and distortion per kilometer in optical fibers used for communications are typically low, signal distortion becomes a communications factor for signals transmitted over increasing transmission distances especially when data is densely compacted using methods such as Dense Wavelength Division Multiplexing (DWDM).
DWDM is a technology that puts data from different sources together on an optical fiber, with each signal carried on its own separate light wavelength. Using DWDM, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light-stream transmitted on a single optical fiber. Thus, in a system with each channel carrying 2.5 GBPS (billion bits per second), up to 200 billion bits can be delivered a second by the optical fiber. DWDM is also sometimes called wavelength division multiplexing (WDM).
Generally, in order to manage and control multiple wavelength optical systems (e.g. DWDM systems), information may be needed at each optical node regarding the wavelength and power of each wavelength transmitted through the system.
Optical power level and integrity monitoring becomes more critical as system topologies become more complex. Systems that used point-to-point transmissions have become increasingly complex by adding and dropping optical wavelengths, or channels, and mesh topologies, with rings of optical fiber lines for backup. Information obtained in monitoring optical transmission data can be used to change the optical parameters of a system (such as the transmitter power), regulate optical module operation parameters, and ease the fault isolation process (for example, in WDM systems). Nevertheless, in many monitoring systems, power is monitored using a different detector for each light wavelength, which can prove expensive for optical fibers carrying many wavelengths and difficult to use for people who need to maintain optical fiber transmission systems.
While monitoring power is very important, it is also important to verify the integrity of the signal carried in the different wavelengths of the DWDM system. That is, that the light traveling in all the DWDM wavelengths are at the proper height and have the right spectral components. Prior art methods for testing optical transmission integrity have very limited data rate capabilities. For example, current methods try to obtain the high data rate testing capability using Monolithic Microwave Integrated Circuits (MMIC) or high speed silicon-germanium chips. Although, the silicon-germanium chips are very fast, they still must be treated as microwave components and inserted into cavities to prevent resonant excitation thus diminishing the advantages of dye casting (i.e., high speed silicon-germanium chips). Thus, prior art systems that use all microwave components start experiencing serious drawbacks as the data frequency increases. For example, current optical network testing systems employing dye casting only operate reliably on data up to ten gigahertz.
Testing the integrity of optically transmitted data requires recovering clock and data from the input signal. In NRZ (Non-Return to Zero) format input data, the clock is not included in the data stream thus it must be recovered from the knowledge that data transitions occur at the clock frequency. Prior art methods are not reliable in recovering the clock at very high frequencies because of the nature of NRZ data. Because there is no clock in the NRZ data stream, there usually are extended periods of time where there is no data transition thus precluding a reliable means for recovering the clock signal from the incoming data stream in prior art systems. In RZ (Return to Zero) format input data, the clock is included in the data stream thus clock recovery is not necessary.
Thus, a method for reliably recovering the clock and data from a high frequency NRZ data stream is desired. Also, a method for reliably testing the integrity of optical transmission components at high frequencies is desired.